The term "dredging" is commonly associated with procedures for excavating relatively large quantities of subaqueous materials. Dredging normally has four principal objectives: (1) to develop and maintain greater depth than naturally exists for canals, rivers, and harbors; (2) to obtain fill to raise the level of low lands and thus create new land areas and improved drainage and sanitation; (3) to construct dams, dikes, and other control works for streams and sea shores; and (4) to recover subaqueous deposits or marine life having commercial value, such as precious metals and minerals, shell fish for food and pearls, coral and sponges, sand and gravel, and fertilizer.
The material to be removed by dredging operations is usually derived from one of two sources or from a combination of both. In harbors at the mouths of rivers, quantities of silt are carried down in suspension and tend, partly because of the deceleration of the flow in the increased waterway available and partly because of the effects of increasing salinity, to be deposited at the mouth, usually the site of harbor works.
In areas in which the deposited silt is highly mobile and accumulates in considerable quantities, it can be economically removed by a suction dredge, which pumps water mixed with silt into hoppers. By adjustment of the capacity of the hopper to the rate of flow from the pump, the water can be made to remain in the hopper long enough to deposit most of the silt. Careful design of the pumping machinery is required to assume a continuous mixture of maximum silt with minimum water.
The early suction dredges generally operated from moored positions in the same manner as bucket-ladder dredges, but a less elaborate system of moorings generally sufficed because the levelling of the seabed could be left to occur naturally through the mobility of the material. A marked advance was achieved by the elimination of much of the lifting and laying of moorings through the development of the trailer suction dredge. The craft has the capacity to dredge while on the move and cruises up and down the waterway or other area, sucking up silt as it goes.
Dredges are characteristically designed to deliver their output either overside into attendant hopper barges or, in the case of self-propelled dredges, into hopper compartments incorporated in their own structure. These compartments are essential in the case of trailing suction dredges, but their value, in other cases, depends on the circumstances and the method of disposal of the spoils. When a long journey to the depositing area is involved, it is generally more economical to leave the dredge continuously at work and remove the spoil in separate barges. When the journey is short and the spoil is to be simply dumped, for which purpose the hoppers are provided with bottoms that fall open, then an economical work cycle between dredging area and spoiling ground, using one craft only, can frequently be established.
Dredged spoil is less and less often disposed of by dumping out at sea, a practice which was once almost universal. Instead, dredging spoils are being used with the reclamation of land from the sea and foreshore. This process has been stimulated by the rise in the value of the land so created and by the discovery that, in many instances, spoil taken out to sea frequently returns. A variety of procedures have been developed for the combined operation of dredging and reclamation. Where the area to be dredged and the area to be reclaimed are in close proximity, as often happens, the whole operation can be carried out by a single suction dredge pumping ashore through a floating pipeline. When, as is more often the case, there is a considerable distance between the two sides, transport in hopper barges is more economical. At the reclamation site, the barges can either be pumped out by a suction reclamation unit, or occasionally can dump their loads on the bottom; from there the material can be pumped ashore by the unit acting as a stationary suction dredge.
Unfortunately, one of the main problems with the reclamation of land by the use of the dredging spoils is the salinity of the dredging spoils. This is particularly the case where the dredging spoils are taken from a salt water or brine environment. The salt contamination of the silt, which is removed in the dredging operations, effectively prevents the use of such soil for effective purposes, such as for agriculture. It is extremely difficult to grow crops on salt contaminated soil. Normally, the dredging spoils will take approximately fifteen to twenty years to become naturally reclaimed. In the meantime, such dredging spoils are essentially dead land, unusable for virtually all purposes.
There are serious costs associated with the unloading of dredging spoils onto land. First, and foremost, there is the cost associated with the land itself. Whenever it is desired to reclaim the dredging spoils by placing them on land (as opposed to being dumped at sea), then an adequate area of land must be found. The land which is used in the dredging operations is often close to the harbor areas and is relatively expensive. Additionally, the continued dumping of the dredging spoils onto the land will effectively kill the previously viable land onto which it is dumped. Alternatively, if an area of land is used for the receipt of dredging spoils, and the spoils are removed from the land and loaded onto a barge, then there are extreme costs associated with the loading of the material onto the barge, for the use of the barge, and for the unloading of the barge. Often, taxes and other fees are required for those that chose to dump the dredging spoils onto the land.
Another important factor is that even when a land area is provided for the receipt of dredging spoils, there is continual leaching of the salt from the containment area. Any leaching of the salt from the containment area will effectively kill the land surrounding the containment area.
There have been various efforts in the past to use organics and fertilizers for the treatment of such dredging spoils. Although such organics are useful for the reclamation of the dredging spoils, the use of such organics and fertilizers is very expensive. The salt-contaminated dredging spoils will rot the organics and the fertilizers and also create methane gas problems. The salt, in the dredging spoils, will effectively kill the nutrients. Any clay particles in the silt of the dredging spoils will tend to attack and hold onto the salt by cation exchange. The reclamation of the dredging spoils requires approximately ten times the amount of fertilizers and organics than would be required for the reclaiming of salt-free dredging spoils.
Even when the dredging operation occurs in fresh water areas, such as rivers, streams, and lakes, there is still the problem of salt contamination. Often, there is a salt runoff that has accumulated on the bottom of the water over a period of time. The silts and clays at the bottom of the body of water will tend to receive any salts that are passed into the fresh water. As a result, the dredged silts will still have structural problems and dispersion difficulties.
In the past, there have no efforts to remove the salt contamination from the dredging spoils. A typical solution to the problem of salt contamination of dredging spoils has been simply to place large amounts of fertilizers and organics onto the dredging spoils. Occasionally, a layer of top soil has been placed over the dredging spoils in an effort to reclaim the area. This is counterproductive in that the top soil is often ruined by its close association with the salt contaminated spoils.
It is object of the present invention to provide a method which is effective for the removal of salt from the dredging spoils.
It is another object of the present invention to provide a method that allows for the reclamation of the dredging spoils into a useful soil.
It is another object of the present invention to provide a method that reduces salt contamination to areas surrounding the dredging spoils.
It is still a further object of the present invention to provide a method that is easy to use and cost effective.
These and other objects and advantages of the present invention will become apparent from a reading of the attached specification and appended claims.